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Determine the Functions of Digestive Enzymes in Breading Down Starch

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Author: Janice VanCleave

In an adult human, food makes about a 30-foot (9-m) trip through the body as it is systematically broken down in preparation for use by body tissues. The pathway begins in the mouth, runs through the esophagus, stomach, and small intestine, and ends in the large intestine.

In this project, you will test for the presence of glucose and starch and determine the function of digestive enzymes in breaking down starch. You will use models to demonstrate the movement of molecules through a semipermeable membrane and the movement of foods through the digestive tract via peristalsis. The absorbency of the intestine due to its structure will also be simulated.

Getting Started

Purpose:   To determine a positive test for starch and glucose.

Materials

  • tablespoon (15 ml)
  • 1% starch solution (see Appendix 2)
  • 3 small baby-food jars (or test tubes)
  • marking pen
  • masking tape
  • Tes-Tape® (glucose enzymatic test strips found at a pharmacy)
  • eyedropper
  • tincture of iodine
  • 1/2 cup (125 ml) of apple juice without added sugar
  • distilled water

Procedure

CAUTION: Keep the iodine out of reach of small children. It is poisonous and is for external use only.

  1. Pour 2 tablespoons (30 ml) of the starch solution into one small jar. With the marking pen, write "Starch" on a piece of masking tape and tape this label to the jar.
  2. Test for the presence of glucose in the starch solution by dipping the end of a 1-inch (2.5-cm) strip of Tes-Tape into the solution. Remove the strip and wait two minutes. Compare the color of the strip to the color chart on the Tes-Tape container.
  3. Record the color of the Tes-Tape and use the color chart to determine the percentage of glucose in the solution (see Figure 26.1).
  4. Test for the presence of starch by adding three drops of iodine to the starch solution, swirl to mix, and record the resulting color.
  5. Pour 2 tablespoons (30 ml) of the apple juice into the second small jar. (Apple juice contains glucose and will be used as a positive test for glucose.) Label the jar, "Glucose."
  6. Dip a strip of Tes-Tape into the apple juice. After two minutes, record its color and percentage of glucose.
  7. Add three drops of iodine to the apple juice, swirl, and record the resulting color.
  8. Pour two tablespoons (30 ml) of distilled water into the third jar. Label the jar, "Control."
  9. Dip a strip of Tes-Tape into the water and record its color and percentage of glucose.
  10. Add three drops of iodine to the water, swirl, and record the resulting color.

Results

Iodine turns the starch solution blue-black; in the apple juice and water, the color is amber. The Tes-Tape remains yellow after being dipped into the water and starch solution; in the apple juice, the tape turns a dark green.

Why?

A positive test for the presence of starch is a blue to blue-black color when iodine is added to the solution. The color is due to the presence of the iodine-starch compound. A positive test for the presence of glucose using the Tes-Tape is a green color. The darker the green, the greater the percentage of glucose in the solution. The control is used to show that water does not change the color of iodine or Tes-Tape.

Function of Digestive Enzymes

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